1. Field of the Invention
The present invention relates to a controller of an injection molding machine, and more particularly, to a controller which maintains a mold clamping force in a toggle type mold clamping apparatus at a constant level.
2. Description of the Prior Art
In a toggle type mold clamping apparatus in an injection molding machine, a movable mold abuts against a fixed mold and then, the movable mold (or movable platen on which the movable mold is mounted) is moved, a tie bar which connects a fixed platen on which the fixed mold is mounted and a rear platen on which the toggle type mold clamping apparatus is mounted with each other is expanded, thereby generating a mold clamping force. When the mold is replaced, the thickness adjustment for adjusting a position of the rear platen is carried out such that a set mold clamping force can be obtained when a crosshead of the toggle type mold clamping apparatus reaches a predetermined position. After that, if the thickness of the mold is not changed, the set mold clamping force can be obtained by moving the crosshead to a predetermined position. Therefore, the mold clamping force is controlled such that it becomes equal to a target mold clamping force depending upon the crosshead position.
During an automatic operation for continuously molding molded products, it is important to maintain the mold clamping force constantly to stabilize the thicknesses of the molded products and outgassing state. During the automatic operation, however, the molding temperature is changed and the thickness of the mold is varied due to this temperature change. Thus, even if the crosshead is positioned to the initial predetermined position, the mold clamping force can not be maintained at the set mold clamping force.
To solve this problem, Japanese Patent Application Laid-open No. 5-200812 discloses a method for correcting the mold clamping force by providing a mold temperature sensor and by carrying out a mold thickness adjustment in accordance with the mold temperature. According to this method, however, since it is necessary to provide a mold clamping force detector such as a mold temperature sensor or a distortion sensor, costs of the apparatus are increased.
Japanese Patent Application Laid-open No. 10-119099 discloses a method in which a tie bar is provided with a mold clamping force detector such as a distortion sensor, and if a mold clamping force detected by the mold clamping force detector corresponds to a set mold clamping force, a motor which drives a toggle mechanism is stopped, and the mold clamping force is feedback-controlled. According to this method, however, since it is necessary to provide a mold clamping force detector such as a mold temperature sensor or a distortion sensor, costs of the apparatus are increased.
Japanese Patent Application Laid-open No. 2000-141012 discloses that a mold clamping force is detected by a mold clamping force detection sensor comprising a load cell or the like, and if the detected mold clamping force exceeds a set tolerance, a support disk of a mold clamping drive source is moved to adjust a die height. However, there is a problem that if the die height is adjusted, a molding cycle becomes long.
Japanese Patent Application Laid-open No. 7-32435 discloses that a position sensor for detecting a position of a mold clamping housing in an opening and closing direction of a mold is provided, and when carrying out mold thickness adjustment, a moving amount of the mold clamping housing from the mold touch to completion of mold clamping operation is stored as a reference position change amount, and a deviation between the moving amount of the mold clamping housing and the reference position change amount is obtained at the time of actual molding operation, and the mold clamping housing is moved based on this deviation. However, since the die height is adjusted, there is a problem that the molding cycle becomes long.
Japanese Patent Application Laid-open No. 4-86209 discloses that in a mold clamping mechanism for driving a toggle mechanism by a servomotor to move a movable mold, thereby closing the mold, a torque of the servomotor is limited in a mold protection section to drive the servomotor under low pressure, and if the mold protection section is completed, a mold torque is limited to such a degree that the tie bar is not extended, the servomotor is driven for a predetermined and after the predetermined time is elapsed (with the mold touched), the servomotor is driven from a position of a crosshead at that time to a position of the crosshead where the set mold clamping force is generated. According to this method, however, since it is necessary to move the mold with low torque to detect a mold touch position for a given time after the mold protection section is completed, and there is a problem that the molding cycle becomes long. Further, the mold thickness exceeds the initially determined mold protection section completion position due to thermal expansion of the mold, and there is a problem that a precise mold touch position can not be detected.